Complete sets of chromosomes are faithfully transmitted to daughter cells during cell division. The molecular mechanism underlying this process is not well understood. The goal of this proposed project is to gain a molecular understanding of the components responsible for chromosome segregation in the yeast Saccharomyces cerevisiae. Using a set of newly identified ipl mutants that are defective in chromosome segregation, genetic and biochemical approaches will be used to address the following questions. 1. What is the role of the IPL1 protein kinase in mitosis? The predicted kinase activity of the newly identified IPL1 protein will be demonstrated biochemically. The endogenous substrates of this kinase will be identified, and the possible role of the IPL1 protein in kinetochore function will be studied by biochemical and genetic methods. 2. What are the roles of the other IPL proteins? The other four IPL proteins defined by mutants will be studied to gain an understanding of their roles in chromosome segregation. This will involve phenotypic characterization of the ipl mutants, molecular cloning and characterization of the IPL genes, and subcellular localization of their gene products. The results from these studies will allow the formulation of specific molecular models to be tested biochemically. 3. What other proteins are involved in chromosome segregation? Additional proteins required for mitosis will be identified by genetic methods, which will also provide information about the possible physical interaction between these proteins. A detailed understanding of the entire chromosome segregation machinery will eventually emerge from the study of these proteins. Because of the conserved nature of many components involved in chromosome segregation, what is learn from these studies in yeast is likely to apply to higher cells where improper chromosome segregation causes a variety of genetic disorders, such as Down's syndrome and spontaneous fetal abortion. It may also play a role in the promotion of neoplasia.